Because of its biodegradability, polylactide [poly(lactic acid)] has become of increasing commercial interest as a substitute for less readily degradable materials such as polyolefins and polyurethanes. The basic technology for making polylactide extends back to as early as 1932 with the work of Carrothers et al. ["Studies of Polymerization and Ring Formation. X. The Reversible Polymerization of Six-Membered Cyclic Esters", American Chemical Society Journal, v. 54, pp 761-772]. Nevertheless, the commercial acceptance of polylactide has been inhibited by the high cost of such polymers as compared to polyolefins and polyurethanes. However, as concerns for the environment become greater, the urgency of using more environmentally friendly materials such as polylactide has become correspondingly greater. To meet this urgency, there exists a substantial need for more economical ways of making poly(lactic acid) [PLA].
The basic process for making PLA involves the dehydration of aqueous lactic acid to form a mixture of oligomers of lactic acid. The oligomers are then subjected to thermocracking to make lactide, that is, the cyclic diester of lactic acid. The lactide is admixed with a ring-opening catalyst and subjected to heat and/or pressure to form PLA.
This general method for making PLA is illustrated by the integrated process disclosed in U.S. Pat. No. 5,142,023 to Gruber et al. and related patents U.S. Pat. No. 5,247,058, U.S. Pat. No. 5,247,059 and U.S. Pat. No. 5,258,488.
Gruber et al. disclose an integrated process for making PLA from aqueous crude lactic acid comprising the following sequential steps:
1. In two stages, evaporating water from the crude lactic acid to form lactic acid oligomers having a molecular weight of 100-5,000 (n=1.1-72);
2. Mixing the oligomers with depolymerization catalyst and thermally cracking the oligomer to form lactide vapor;
3. Removing lactide vapor from the thermal cracking zone, condensing it and fractionally distilling the lactide condensate to form a "purified" lactide; and
4. Reacting the "purified" lactide to form PLA.
It is recognized by those skilled in the art that water and/or its reaction products with materials such as lactic acid and lactic acid oligomers should be kept at very low concentrations in order that high molecular weight polymers can be made therefrom. For example, see British Polymer Journal, Vol. 23, No. 3, p. 235-240 (1990), which teaches that the content of free carboxylic groups should not exceed 0.8 meq/g (800 meq/kg). Nevertheless, it has been found that even such modest amounts of acids are much too high for the manufacture of high purity lactic acid polymers.
However, the Gruber et al. process is based on the premise that the lactide from the oligomer cracking unit, if fractionated by distillation, will be suitable for polymerization without further treatment such as crystallization or solvent extraction, both of which have been suggested for this use in the prior art. However, it has been found that the process taught by Gruber et al. has several shortcomings which make it unsuitable for the manufacture of high quality PLA.
In particular, following the teaching of the Gruber process results in (1) a manufacturing sequence which requires extensive time for polymerization, (2) excessive equipment costs and (3) products therefrom having inadequate properties for many applications. Many of these polymer quality concerns arise from the fact that the process as taught fails to recognize the cause and effect of the many and varied side reactions which adversely affect polymer quality.
While the Gruber et al. process purports to advance the technology of making PLA, it nevertheless falls considerably short of teaching a practical process which can make PLA readily available in commercial quantities with the high quality that is needed for the widespread use of PLA in consumer applications. Therefore, there remains a substantial unmet need for a practical route for making high quality lactide which is suitable for making PLA by ring-opening polymerization thereof.